1. Field of the Invention
This invention relates to apparatus for testing unpopulated printed circuit boards, and, more particularly, to such apparatus including a generic pattern of test points, not specific to the particular type of card being tested.
2. Background Information
A number of methods are used to test unpopulated printed circuit boards for shorts and opens. In general, boards are generally tested either through the use of a specialized fixture having a spring-loaded probe point at each circuit pad to be contacted during the test process, called a "bed of nails" circuit tester, or through the use of a flying-probe circuit tester, which has a relatively small number of probe points, typically one to four, moving rapidly among the points to be tested.
The general method used with either of these types of test probes is to apply a test voltage to only one of the circuit pads and to look at each of the other circuit pads to determine if the test voltage has been conducted to it through the circuit card. That is, each pair of circuit pads on the circuit board is checked to determine if there is a circuit connection therebetween. In a board having N test points, the number of tests required by this method is given by T in the following equation: ##EQU1##
The results of this test are compared with data for a circuit board of the type being tested which is known to be good. If one or more of the circuit connections which are supposed to be present is in fact absent, the circuit board is rejected as having one or more open conditions. On the other hand, if one or more of the circuit connections which is supposed to be absent is in fact present, the circuit board is rejected as having one or more shorted conditions. If neither of these failure conditions is present, the circuit board is accepted as having passed the test process
A particular problem with the "bed of nails" testing approach is that a complex test fixture must be built for each type of circuit board to be tested. Furthermore, the close spacings of circuit features, including circuit pads, on modern circuit boards makes the construction of such test fixtures difficult, or even impossible, due to the space requirements of the spring-loaded pins. While the method of flying probes allows the same probing apparatus to be used for many different types of circuit boards, the time required for probe movement makes this method much to slow for many circuit testing applications. Also, both methods require the precise alignment of the individual circuit boards with the test apparatus so that circuit boards are contacted correctly. This alignment is becoming increasingly difficult as circuit pad spacings are reduced.
Thus, what is needed is a method allowing the use of a single, generic test fixture for the application of test voltages to test pads in a large number of different types of circuit cards, including cards with close pad spacings. Furthermore, what is needed is a circuit testing method alleviating the need for accurate placement of the circuit board within a testing fixture.
A number of U.S. Patents describe testing fixtures having flexible membranes with printed circuit pads arranged to contact particular features on circuits to be tested. This method provides for the placement of test pads at much closer spacings than those achievable with spring-loaded test points 20, allowing the fixtures to address the problems of testing integrated circuit substrates. Also, in many instances, such test fixtures are less expensive to build than the traditional "bed-of-nails" fixtures, with easily-replaceable membranes being used. Such patents include U.S. Pat. Nos. 4,906,920, 5,148,103, 5,313,157, 5,355,079, 5,436,568, 5,461,326, and 4,469,072. Nevertheless, this method requires specialized fixtures for the various different types of circuit substrates being tested, with each fixture having test pads located at the specific locations of circuit pads on the substrate to be tested. Furthermore, the circuit being tested must be located very accurately relative to the test fixture, so that each circuit pad on the circuit being tested contacts the appropriate pad of the test fixture and no adjacent pads of the test fixture.
U.S. Pat. No. 5,506,510 describes the use of a generic probe fixture in an automatic circuit board tester. The fixture includes a high-density array of probe pads, which are sized and spaced apart to ensure contact with all of the test points on the device under test. The width of the probe pads is made smaller than the known minimum separation of the test points, and the separation of the probe pads is made smaller than the known width of the test points. While this fixture structure results in the possibility that more than one probe pad will contact the same test point, each of the test points makes contact with at least one probe pad.
The method of U.S. Pat. No. 5,506,510 requires an extremely high density of probe pads for typical circuit testing applications. This density requires a very large number of associated circuits, such as multiplexers, within the test equipment, increasing the cost of the equipment and slowing the testing process. This density is generally too high to allow the placement of the test pads and associated circuit lines on a flexible membrane, since the resulting density of the circuit lines is too high to allow the manufacture of a practical flexible printed circuit. The apparatus described in the patent includes a rigid assembly of test pads, from which individual wires extend, with reliance being placed on the deflection of a flexible circuit assembly being tested to make electrical contact at all of the test points. A vacuum arrangement is described for causing the deflection of the flexible circuit assembly in this manner. This requirement for sealing and deflection places limitations on the types of circuit assemblies which can be tested within the apparatus.
What is needed is a method providing a generic test fixture having a pattern of probe pads which is of a low enough density to allow placement of the pads and associated circuit lines on a flexible membrane. Such a test fixture could be used to test circuit substrates of a wide variety, including both rigid and flexible substrates without a need for precisely positioning the substrate being tested relative to the test fixture. Furthermore, such a test fixture would provide the advantages associated with an easily replaceable fixture portion contacting the circuits being tested.
A number of techniques have been developed and described for recognizing patterns in the non-analogous field of optical pattern recognition. Descriptions of various of these techniques are provided, for example, in U.S. Pat. No. 3,614,736, which discloses a pattern recognition system which will recognize patterns irrespective of their translation, rotation, or scale change. Input data may be provided by a scanner or other suitable data source. Means for calculating the center of gravity, or alternately the autocorrelation function are provided which can be employed, so that the data can be transformed for an actual or simulated annular or equivalently radial scan, with exponential spacing along radii. Alternatively, a straightforward raster scan may be employed for recognition which is invariant to translation only. The output is then processed in means for cross-correlating with known patterns.
What is needed is a means for gathering circuit test data, relative to short and open circuit conditions, through the use of a generic fixture, not specific to the type of circuit being tested, and cross correlating such data with similar data measured using a known good circuit.